Recently, quality of a cold rolled steel sheet for forming and its production technologies have made remarkable progress, and particularly, a Rankford value (hereinafter referred to as the "r value") representing deep drawability when cold has become as high as 1.7 or more.
As a technology for obtaining such high performance, Japanese Examined Patent Publication (Kokoku) No. 44-18066 discloses a technology which fixes and stabilizes C by adding 0.02 to 0.5% of Ti to a steel containing 0.001 to 0.02% of C and not greater than 0.015% of 0.
Japanese Examined Patent Publication (Kokoku) No. 3-54186 discloses a method which reduces both C and N contents to at most 0.005% and adds Ti and Nb, and Japanese Examined Patent Publication (Kokoku) No. 53-12899 discloses a technology which minimizes the C and N contents and adds Ti, Nb and B for fixing them.
These prior art technologies are based on the common technical concept of reducing the C and N contents as much as possible and obtain a cold rolled steel sheet having excellent non-ageability and excellent cold formability.
Re-utilization of steel scrap which is generated in large quantities everywhere has become a very important problem at present. To re-utilize the scrap, it is common to arc melt the scrap by an electric furnace, or the like. In this case, the N content in the steel reaches a high level of 60 ppm or more because N.sub.2 in the air comes into the steel.
When the N content is great, formability drops remarkably. Particularly in a high nitrogen region having an N content of 60 ppm or more, forming involving large deformation such as deep drawing becomes hardly possible. Means such as vacuum degassing must be employed in order to reduce this nitrogen content, and the production cost unavoidably increases.
Japanese Unexamined Patent Publication (Kokai) No. 57-26124 can be cited as a prior art reference which discloses a production technology of a cold rolled steel sheet having a high N content. This technology continuously anneals a cold rolled steel sheet having a specific C, Mn and Al content and containing 30 to 200 ppm of N at a predetermined temperature, and obtains a cold rolled steel sheet having excellent bake-hardenability. Because this cold rolled steel sheet contains a large quantity of N, a content of free N (the balance obtained by subtracting N bonded to Ti, Nb, etc., from the total N content) can be secured. This free N exhibits bake-hardenability. This reference discloses also that when this steel sheet is allowed to contain P, Si and Ti, its strength can also be improved. Nonetheless, Japanese Unexamined Patent Publication (Kokai) No. 57-26124 does not at all disclose a technology for securing formability of a high N steel.
As means for solving this problem, the inventors of the present invention invented, and filed a patent application for, a high N content hot rolled steel sheet having high strength and high formability and a production technology thereof in Japanese Patent Application No. 4-292352. This reference allows a high N (50 to 150 ppm) steel to contain C so that the carbon equivalent becomes 0.1 to 0.45%, secures an area fraction of pearlite of at least 5% so as to secure uniform elongation after cold forming, and disperses TiN of which sizes 1 .mu.m or more in a weight proportion of 0.0008 to 0.015% so as to obtain a hot rolled steel sheet having a tensile strength of 34 kgf/mm.sup.2 or more.
However, this reference does not at all suggest formability of a high N cold rolled steel sheet.
When re-utilizing a scrap and using an electric furnace for this re-utilization, the present invention provides a cold rolled steel sheet having excellent formability, though it has a high N content, and a production method which does not require processing such as vacuum degassing.